Video cameras have been used to record not only a scene viewed by a pilot on a mission but also the head-up display viewed by the pilot as well (FIG. 1). The camera 1 is positioned on the pilot's side of the combiner 2 (a semi-reflecting, semi-transmitting element) which reflects symbols generated in the head-up display into the pilot's line of sight. The recording may then be used for instruction purposes when the pilot returns to base.
The dynamic range, however, of a video camera is less than that of the human eye, with the result that the HUD symbology may appear too faint on the recording. To alleviate this, it has been proposed to use filters which restrict all colours except those of the HUD symbology, either on the camera or on the combiner, but in either case adequate contrast is not obtained without excessive attenuation of the view of the outside world.
It has also been proposed (GB-A-2065053) to combine electronically signals from a camera for imaging the scene viewed by a pilot and signals for generating a HUD.